Rust is a general name for a complex of oxides and hydroxides of iron, which occur when iron or some alloys that contain iron are exposed to oxygen and moisture for a long period of time.
Over time, the oxygen combines with the metal forming new compounds collectively called rust. Although rust may generally be termed as "oxidation", that term is much more general and describes a vast number of processes involving the loss of electrons or increased oxidation state, as part of a reaction. The best-known of these reactions involve oxygen, hence the name "oxidation".
The terms "rust" and "rusting" only mean oxidation of iron and its resulting products. Many other oxidation reactions exist which do not involve iron or produce rust. But only iron or alloys that contain iron can rust. However, other metals can corrode in similar ways.
The main catalyst for the rusting process is water. Iron or steel structures might appear to be solid, but water molecules can penetrate the microscopic pits and cracks in any exposed metal. The hydrogen atoms present in water molecules can combine with other elements to form acids, which will eventually cause more metal to be exposed. If chloride ions are present, as is the case with saltwater, the corrosion is likely to occur more quickly. Meanwhile, the oxygen atoms combine with metallic atoms to form the destructive oxide compound. As the atoms combine, they weaken the metal, making the structure brittle and crumbly.
Fe Fe Fe 😂